Many applications require reliable, stable and efficient spectrally-pure high-power light sources. For example, image projection systems require light sources which exhibit these characteristics and deliver in excess of 1 Watt average power. These light sources should be inexpensive to produce and they need to generate output frequencies in the blue range and in the green range. For other applications light in the UV range is required.
The prior art teaches various types of light sources for generating light in the visible and UV ranges, including frequencies corresponding to blue and green light. A number of these sources rely on a nonlinear frequency conversion operation such as second harmonic generation (SHG) to transform a frequency outside the visible range, e.g., in the IR range, to the desired visible or UV frequency. For example, U.S. Pat. No. 5,751,751 to Hargis et al. teaches the use of SHG to produce deep blue light. Specifically, Hargis et al. use a microlaser which has a rare earth doped microlaser crystal and emits light at about 914 nm to drive SHG in a crystal of BBO producing output at about 457 nm.
U.S. Pat. No. 5,483,546 to Johnson et al. teaches a sensing system for high sensitivity spectroscopic measurements. This system uses a passively Q-switched laser emitting light at a first frequency. The light from the laser is transmitted through a fiber and converted to output light at a second frequency in the UV range. The conversion is performed by two frequency doubling crystals disposed far away from the Q-switched laser.
U.S. Pat. No. 6,185,236 to Eichenholz et al. teaches a self frequency doubled Nd:doped YCOB laser. The laser generates light of about 400 mW power at about 1060 nm and frequency doubles it with the aid of a frequency doubling oxyborate crystal to output light in the green range at about 530 nm. Eichenholz et al. combine the active gain medium and the frequency doubler in one single element to produce a compact and efficient light source.
In U.S. Pat. No. 5,909,306 Goldberg et al. teach a solid-state spectrally pure pulsed fiber amplifier laser system for generating UV light. This system has a fiber amplifier in a resonant cavity and an acousto-optic or electro-optic modulator incorporated into the cavity for extracting high-peak-power, short-duration pulses from the cavity. These short pulses are then frequency converted in several non-linear frequency conversion crystals (frequency doubling crystals). The addition of the modulator into the cavity for extracting the pulses and placement of the fiber amplifier within the resonant cavity renders this system very stable and capable of delivering a spectrally-pure pulse. Unfortunately, this also makes the system too cumbersome and expensive for many practical applications such as display systems.
U.S. Pat. No. 5,740,190 to Moulton teaches a three-color coherent light system adapted for image display purposes. This system employs a laser source and a frequency doubling crystal to generate green light at 523.5 nm. Moulton's system also generates blue light at 455 nm and red light at 618 nm by relying on frequency doubling and the nonlinear process of optical parametric oscillation.
Unfortunately, the light sources described above and various other types of light sources taught by the prior art can not be employed to make stable, low-cost, efficient sources of light delivering 1 Watt of average power for display applications. This is in part due to the fact that frequency conversion, e.g., frequency doubling in crystals, is not a very efficient operation. If the frequency doubling crystal had extremely high non-linearity, then low power continuous wave (cw) lasers could be efficiently doubled to generate output power levels near 1 Watt. However, in the absence of such frequency doubling crystals high-peak-power, short pulse lasers have to be used to obtain frequency doubled light at appreciable power levels. It should also be noted that providing such high-peak-power short pulses adds complexity to the design of the light sources and introduces additional costs.
U.S. Pat. No. 5,394,413 to Zayhowski addresses the issue of efficient frequency doubling by using a passively Q-switched picosecond microlaser to deliver the pulses of light. Such pulses can be efficiently converted, as further taught by Zayhowski in a frequency-doubling crystal. Devices built according to Zayhowski's teaching operate at relatively low average power levels and low repetition rates. Attempts to increase these parameters by pumping the microchip harder will cause multiple transverse-mode operation leading to degradation of beam quality and also incur increased pulse-to-pulse noise. Hence, Zayhowski's devices can not be used in applications such as projection displays, which require high average power and high repetition rates and good beam quality.
Hence, what is needed is a stable and efficient source of light in the blue and green ranges which can be used in a projection display.